1. Field of the Invention
The present invention relates to a lighting technology, particularly to a high-efficiency lighting device and a method for fabricating the same.
2. Description of the Related Art
The light emitting diode, abbreviated as LED, was usually used as the component of indicators or display panels before. Owing to technical advance, LED is also used as the light source now. LED can convert electric energy into light energy efficiently and has a service life of as long as tens to hundreds of hours. LED has nothing to do with the fragility of the traditional light bulb but has much higher power efficiency than the traditional light bulb. LED further has the following advantages over the traditional light bulb: environmental friendliness (mercury free), small size, low temperature, directivity, less light pollution, and abundant color gamut. Therefore, LED has a high potential to replace the traditional lighting devices. In the recent years, LED has been applied to daily living, such as house illuminators, backlight devices, street lights, and vehicle lights. LEDs made of different materials respectively emit different-color lights. For example, a quaternary InGaAlP LED emits yellow light, and a ternary GaAsP LED emits red light, and a binary GaN LED emits blue light.
GaN is a direct-gap semiconductor material having a wider energy band gap (about 3.48 ev) and a higher photoluminescence efficiency. GaN has been extensively studied by scientists recently. As early as in 1932, the GaN chip had been achieved with a high-temperature synthesis method by W. C. Johnson, et al. However, a larger-area GaN did not appear until 1962. In 1962, H. P. Muruska, et al., successfully grew a larger-area GaN epitaxial film on a sapphire substrate with a HVPE (Hydride Vapor Phase Epitaxy) technology and found GaN has a direct band gap of 3.4 ev. As there are crystalline mismatch (as high as 16.1%) and difference of thermal expansion coefficients between GaN and sapphire, GaN epitaxial film has a dislocation density of as high as 109-1010/cm2, which is much higher than the dislocation densities of other nitride semiconductor materials. Therefore, a high-quality GaN epitaxial material was hard to obtain. Table.1 shows the lattice constants of common nitride semiconductor materials.
In 1983, S. Yoshida, et al., formed an aluminum nitride (AlN) layer as a buffer layer on an aluminum oxide substrate at a high temperature and then grew GaN on the aluminum nitride to obtain a higher-quality GaN crystal. In 1986, Amano, et al., (a research team of Professor Isamu Akasaki, Nagoya University) successfully grew a transparent and surface crack-free GaN film on a low-temperature AlN buffer layer with an MOCVD epitaxial technology. Later, Akasaki, et al., using X-ray diffraction spectrums and photoluminescence frequency spectrums, proved that the GaN film grown on the low-temperature buffer layer has perfect crystallinity, and that the concentration of the intrinsic defect-induced donors is reduced to 1×1015/cm3, and that electronic mobility is increased by a scale of 10. In 1992, Dr. Nakamura in Nichia Co. Japan, using an annealing technology, successfully activated the GaN film grown on the low-temperature buffer layer. In 1995, Nakamura successfully achieved a blue-light GaN LED and a green-light GaN LED. In 1996, Nakamura achieved a white-light LED, wherein a blue-light InGaN LED (with a wavelength of 460 nm-470 nm) is used to excite the yellow fluorescent material of YAG (yttrium-aluminum garnet): Ce3+ with 5d-4f transition to obtain white light. Thereafter, the technology of nitride semiconductor materials expanded rapidly with the persistent development of the crystal-growth technology and the optoelectronic technology. Considering the vast field in illumination application, the blue-light LED, true-color LED and white-light LED are expected to continuously have great annual growth in the global market and finally achieve a dominant role in lighting instruments.
Owing to the patents, only few manufacturers can fabricate and sell blue-light LED epitaxial chips, such as Cree and LumiLeds in US and Nichia and Toyoda Gosei in Japan. The fact considerably impairs the endeavor of Taiwan companies to develop the market of blue-light LED. Besides, high price also impairs the popularization of blue-light LED. The high price of blue-light LED is partially attributed to the high price of the crystal-growth sapphire substrate. If a silicon substrate can be used as a crystal-growth substrate, the cost of blue-light LED will be effectively reduced. Thus, using a cheap silicon substrate to fabricate a high-quality LED epitaxial chip has become a problem the manufacturers have to overcome.
In the conventional technology for fabricating LED epitaxial chips, III-V group semiconductor materials are epitaxially grown on a substrate layer-by-layer. The conventional technology has the problem that the III-V group semiconductor materials have to match the substrate material in the crystalline structures thereof. Currently, the GaN LED epitaxial chips are usually grown on the sapphire substrate. As mentioned above, the two materials have crystalline mismatch (up to 16.1%) and difference of thermal expansion coefficients therebetween. Recently, many research teams have developed various technologies to solve abovementioned problems. However, most of the related patents are possessed by US or Japan manufacturers. Therefore, overthrowing the conventional LED epitaxial technologies and breaking through the existing patents are the problems the other manufacturers are eager to overcome.
Accordingly, the present invention proposes a high-efficiency lighting device and a method for fabricating the same to break through the bottleneck, whereby III-V group semiconductor materials can be grown on an arbitrary substrate in a high quality.